Foamed polypropylene sheet having improved appearance and a foamable polypropylene composition therefor

ABSTRACT

Foamable polypropylene composition comprising a polypropylene resin having an D1238L melt flow rate of from about 0.5 to about 30 g/10 min, and a method for extruding rigid, foamed polypropylene sheet with improved surface appearance having a density in the range 0.4 to about 0.8 g/cm 3 .

CROSS-REFERENCE TO RELATION APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/098,958, filed Sep. 3, 1998, U.S. ProvisionalApplication No. 60/122,129, filed Mar. 1, 1999, and U.S. ProvisionalApplication No. 60/128,173, filed Apr. 6, 1999.

BACKGROUND OF THE INVENTION

[0002] This invention relates to polyolefins, and more particularly toimproved expanded or foamed compositions comprising propylene polymers.Still more particularly, the invention relates to rigid or semi-rigidpolypropylene foam sheet having improved surface appearance.Polypropylene foam sheet according to the invention is readilythermoformable into shaped articles that are particularly useful inrigid and semi-rigid packaging and in fabricating trays, plates,containers and other articles used in food service applications.

[0003] Polystyrene has found wide acceptance for use in food serviceapplications because of its good rigidity and shape retention and, asfoam sheet, it is readily molded and thermoformed. However, polystyrenearticles suffer from low service temperature, and generally are fragileand lack chemical resistance. The food service and packaging arts havelong sought alternative materials that do not have these undesirablecharacteristics.

[0004] Polyolefin resins are widely known for their ease of fabricationand are found in a great variety of applications. Considerable efforthas been expended in recent years to develop rigid expanded or foamedpolyolefin sheet as a replacement for styrene foams, particularly foruse in food service applications. Polyethylene resins have moderatestrength and high toughness, with softening temperatures in the range offrom 105° to 140° C. Such resins are suitable for foam extrusion, givingattractive, low density, thermoformable foam sheet with good lowtemperature properties. Foam polyethylene sheet and molded foam articleswith excellent surface appearance are readily produced from low densitypolyethylene resins and so have found wide acceptability for use in avariety of packaging applications as well as in food serviceapplications. However, polyethylene foams generally are soft andflexible and have poor heat resistance, and thus may find limitedacceptance for food service uses requiring rigidity, and where contactwith hot foods is contemplated.

[0005] Propylene polymers, or polypropylene resins, are particularlynoted for their good heat resistance and mechanical properties, andresin formulations based on polypropylene are supplied to meet thedemands imposed by a variety of structural and decorative uses in theproduction of molded parts for appliances, household goods and autos.Impact modified polypropylene and elastomeric ethylene-propylenecopolymers have found application in automotive applications includinginterior trim as well as in exterior parts such as bumper facia, grillcomponents, rocker panels and the like. Polypropylene resins have thethermal and chemical resistance to withstand exposure to the widevariety of environments and are easily molded at a cost far below thatof metal stamping to provide parts that will not rust or corrode and areimpact resistant, even at low temperature.

[0006] A number of processes for producing polypropylene foam have beendisclosed and are well described in the art, including for example themethods disclosed in U.S. Pat. No. 5,180,571 to J. J. Park et al. andthose set forth in the references cited and summarized therein. The Parket al. patent is directed to the extrusion of polypropylene to providefoam sheet having a low density generally in the range of from 0.04 toabout 0.44 g/cm³.

[0007] At the surface of extruded foamed polypropylene sheet theregenerally may be found a layer consisting substantially of crystallinepolypropylene (PP). This surface layer or skin is important to partappearance and surface hardness. The thickness and crystallinity of thePP surface layer that forms depends in part upon extrusion conditionsincluding die temperature and cooling rates, and upon annealing. ThePark et al. patent is directed to the extrusion of polypropylene toprovide foam sheet with a smooth surface skin and a uniform cellstructure. According to the teachings of Park et al. it is necessary touse high melt strength, high melt elasticity polypropylene with aparticularized combination of molecular and rheological characteristicsincluding bimodal molecular weight distribution and a minor componentthat is highly branched to produce foam sheet having a smooth surfaceskin and a uniform cell structure. Patentees provide comparisons showingthat low density foam sheet extruded using conventional or genericpolypropylene resins, further characterized as polypropylene resins withmonomodal molecular weight distributions and an absence of significantbranching, generally have roughened sheet surfaces and non-uniformmicrocellular structure and are unacceptable for commercial use.

[0008] The surfaces of extruded polyolefin foam sheet generally lack thesmooth, shiny, uniform and substantially unblemished surfaces observedwith extruded styrenic foam sheet, particularly including higher densityABS foam sheet. For example, surface roughness is commonly encounteredwhen extruding polyethylene foam sheet, and lack of uniformity in cellstructure and distribution at the surface is visually more readilyapparent because of the transparent nature of unfilled polyethylene.Sensible surface roughness, that is, roughness that can be sensedtactilely, may be reduced by contacting the lower melt temperaturepolyethylene sheet with a polishing roll during the extrusion process togive a smooth, more even surface. The surface imperfections that remainare mainly visible density variations and are generally uniformlydistributed, providing a textured or marbleized surface appearance thatis pleasing and generally acceptable.

[0009] Rigid polypropylene foam sheet obtainable by the processescurrently known and practiced in the art continues to be somewhatlacking in surface appearance characteristics. Characteristically,polypropylene foam sheet extruded with conventional processes and usingconventional or generic polypropylene resins will have regularly spacedmarkings in the form of alternating bands or corrugation-like markingsextending the length of the sheet in the machine direction. In light,low-density foams obtained from conventional polypropylene resins,particularly soft, flexible foams having a densities of 20 lb/ft³ (0.3g/cm³) and lower, these bands may have the form of a regularly spaced,wave-like or sinusoidal distortion, forming a corrugated sheet. Thebands or corrugations become less pronounced for rigid foam sheet and,particularly at higher foam densities, are seen as surface flaws orappearance defects that take the form of linear, valley-like surfacedepressions along the machine direction.

[0010] The surface roughness of sheet extruded using these highermelting resins is more difficult to smooth adequately using a polishingroll. Moreover, the imperfections and visible density variations foundin the surfaces of extruded polypropylene foam sheet are often notuniformly distributed over the surface, and are generally quite visible,even for sheet that otherwise is tactilely smooth. In rigid, higherdensity foams such as are desirable for the production of food servicearticles the defects more often appear as a pattern of alternatinglinear bands of high and low foam density, characterized by readilyvisible variations in translucence and surface gloss, possibly includingsurface voids, bubbles, streaks and uneven color. Such flaws may bewithout significant effect on the mechanical properties of the foam, andgenerally do not affect the performance of finished goods fabricatedfrom such foam. However, in consumer goods, food packaging and the like,these visible surface defects and related cosmetic flaws are highlyundesirable, thus limiting acceptance of rigid polypropylene foam sheetby the industry.

[0011] Coextrusion of multilayer sheets having solid outer skins and afoamed core has been disclosed in the art and is widely used to overcomesurface appearance problems encountered in the production of a varietyof prior art foam sheet materials including those made from polystyreneand ABS. Foam core sheet, provided with a shiny or glossy unfoamedsurface layer formed of the same or another resin, may be improved inresistance to surface abrasion and cuts and have a superior appearance.The more rigid skin serves to stiffen the foam structure, allowing alighter and thinner structure while attaining maximum bending stiffness.Foam sheet coextrusion processes are well described in the art for usewith a variety of resins such as polystyrene and ABS, and methods havebeen recently disclosed for use in the coextrusion of multilayered foamsheet comprising polyolefins including polypropylene. Coextrusionprocesses suffer the disadvantage of generally requiring more costlyfeedblocks, dies and related machinery having a more complicated design,thereby increasing the complexity of the operation and raising the costof producing such foam sheet.

[0012] Thus, there continues to be a need for rigid, foamedpolypropylene sheet comprising conventional generic polypropylene resinswith reduced visible surface defects and related cosmetic flaws andhaving the attractive, defect-free surface appearance necessary foracceptance in the food service and packaging industries, and forpolypropylene compositions comprising conventional generic polypropyleneresins that are suitable for use in the extrusion of such foam sheet.

SUMMARY OF THE INVENTION

[0013] The invention pertains to the production of rigid, foamedpolypropylene sheet significantly improved in surface appearance. Moreparticularly, the invention relates to improved foamable polypropylenecompositions comprising a propylene polymer, a crystallizationnucleating agent, a bubble nucleating agent, and a blowing agent for usein the production of rigid, foamed polypropylene sheet having a highdensity, greater than about 0.45 g/cm³, with excellent strength andthermal insulating properties. When extruded employing the improvedextrusion apparatus and die according to the further teachings set forthherein, the resulting high density, foamed polypropylene sheet may becharacterized as having a more uniform cellular structure together withan improved surface appearance including significant reduction in thesurface banding, corrugation and related visible flaws commonlyencountered in extruded, high density, rigid foam sheet; the inventionthus may be further characterized as directed to improved extruded,rigid, foamed polypropylene sheet.

[0014] Rigid, foamed polypropylene sheet according to the invention hasa low average surface roughness uniformly distributed over the surfaceof the foam sheet, and a substantial absence of corrugation and surfacebanding. When molded or otherwise thermoformed, the invented foam sheetwill afford rigid or semi-rigid articles having improved surfaceappearance while retaining a good balance of mechanical propertiesincluding stiffness and toughness. The invention thus may also becharacterized as directed to molded articles having improved appearancecomprising expanded or foamed polypropylene.

BRIEF DESCRIPTION OF THE DRAWING

[0015]FIG. 1 is a diagrammatic, perspective view, partially in phantom,of a typical coathanger-type sheet extrusion die.

[0016]FIG. 2 is a sectional view of a prior art polyolefin sheetextrusion die, taken along line A--A of FIG. 1.

[0017]FIG. 3 is a sectional view, taken along line A--A of FIG. 1,showing an improved sheet extrusion die according to the invention.

[0018]FIG. 4 is a sectional view, taken along line A--A of FIG. 1,showing an alternative embodiment of the improved sheet extrusion dieaccording to the invention.

[0019]FIG. 5 is a fragmentary, enlarged sectional view showing thedetail of the die land portion of the embodiment of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0020] The particular advantage of the invention as disclosed herein isthat high density foam sheet having excellent surface appearance may beprovided using any of the great variety of commercially availablepolypropylene resins, and without resorting to specialty resin gradesand blends. The polyolefin compositions useful in the extrusion ofthermoformable, rigid, foamed polypropylene sheet according to theinvention will preferably comprise a substantially linear polypropylenehomopolymer, or a copolymer of propylene and a minor amount, up to about30 wt. %, more preferably up to about 20 wt. % of an alpha-olefin whichmay have up to 6 carbon atoms. The polymer may be syndiotactic orisotactic, however it is preferable to use an isotactic polypropylenehomopolymer having an isotactic index of greater than 0.85, morepreferably greater than 0.92, the articles obtained from saidhomopolymer having better physical properties. The melt flow index ofthe polymer will be from about 0.3 to about 10, preferably from about1.0 to about 4.0 g/10 min., determined according to ASTM D1238,Condition L. Such polymers are readily prepared by a variety ofcatalyzed polymerization processes well known in the art, includingprocesses employing Ziegler-Natta catalysts and those based onmetallocene catalysts.

[0021] A wide variety of extrusion grade, film-forming, substantiallylinear polypropylene resins without significant branching having anessentially monomodal molecular weight distribution and the requisiteMFR are readily available in the trade and most will be found useful forproviding foam sheet having improved surface appearance according to theteachings of this invention.

[0022] According to the teachings of the prior art, it has beengenerally believed that high melt strength resin formulations arerequired in order to successfully extrude foamed polypropylene havinggood cell structure and acceptable surface appearance, and the art hasdeveloped specialty formulations comprising particular grades ofpropylene resins having particularly defined molecular weight andrheological properties including a bimodal molecular weight distributioncomprising a highly branched minor component for these uses. Blendcompositions having the requisite melt strength have also beenformulated using polypropylene that has been modified, for examplethrough crosslinking, or with particular polymeric additives, highlybranched olefin polymers or the like. Although these specialty resingrades and resin formulations may also be found suitable for use in thepractice of this invention, foam sheet having improved surfaceappearance may be readily produced from readily available generic gradesof polypropylene, i.e. propylene polymer resins with monomodal molecularweight distributions and without a significant level of branching, hencesuch specialty resin compositions are not required.

[0023] As disclosed and described in the art, foamable polypropylenecompositions will further comprise a blowing agent and a crystallizationnucleating agent.

[0024] The blowing agent may be of the type well known and widely usedfor the production of expanded polystyrene and polyolefins includingpolypropylene, including organic blowing agents such as, for example,azodicarbonamide, diazoaminobenzene, azo-bis-isobutyronitrile andanalogs thereof, and inorganic blowing agents such as, for example,ammonium carbonate, sodium bicarbonate and the like. Physical blowingagents such as nitrogen, carbon dioxide and other inert gases and agentsthat undergo phase change from liquid to gas during the foaming processsuch as chlorofluorocarbons (CFC), HCFC, low boiling alcohols, ketonesand hydrocarbons, are also known for these uses and may also be founduseful in the practice of this invention. The blowing agent may furthercomprise one or more additives to reduce its decomposition temperature.

[0025] The amount of blowing agent to be used depends on its nature andon the desired density for the expanded polypropylene and will beselected according to practices well understood by those skilled in theresin formulating art. Generally, blowing agents are available to thetrade in the form of concentrates; the concentrates will be added to theformulation at levels that will provide from about 0.2 to about 10 wt. %active foaming agent, preferably from about 0.4 to about 5 wt. % activefoaming agent, based on total weight of the formulation. The amounts ofphysical blowing agents such as liquid blowing agents and inert gasesneeded to provide the desired foam densities may readily be determinedaccording to common commercial practice.

[0026] As set forth in the art, a crystallization nucleating agent isprovided to increase the number of crystallization nuclei in the moltenpolypropylene, thereby increasing the crystallization speed andpromoting crystallization from the melt, solidifying the resin at ahigher temperature. Generally, non-nucleated polypropylene will begincrystallizing at around 120° C. with a peak in crystallization rate near110° C. Nucleated polypropylene resins may start to crystallize attemperatures as great as about 135 to 140° C., with a peak around 130°C. Nucleated resin will solidify rapidly with improved melt strength tothereby reduce sag in the extruded foam sheet. The crystallizationnucleating agent will generally be used in an amount of from about 0.01to about 0.5 wt. %, preferably from about 0.05 to about 0.3 wt. %.Examples of such agents disclosed in the art and employed for improvingthe crystallization speed include organic sodium phosphates such assodium bis(4-tert-butyl-phenol)phosphate, sodium benzoate and mixturescomprising a monocarboxylic aromatic acid or a polycarboxylic aliphaticacid and a silicate or an alumino-silicate of an alkali or alkalineearth metal. The use of organic sodium phosphates as crystallizationagents is disclosed in the art, for example in U.S. Pat. No. 4,596,833.

[0027] Other agents disclosed in the art for improving melt strength ofpolyolefins include sorbitol, dibenzilidene sorbitol and relatedcompounds. These agents have been described in the art as networkingagents for use in modifying the low shear melt viscosity and low shearmelt strength of polyolefins and as crystallization nucleating agents.However, as will be seen, such networking agents are ineffective inproviding high density, rigid, foamed polypropylene sheet according tothe invention.

[0028] Further modifiers and additives for foamable polypropylenecompositions are also disclosed and described in the art. Such modifiersand additives may be employed in amounts according to the commonpractice in the art, including lubricants, coloring and/or dryingagents, fire-proofing agents, thermal and UV stabilizers, antioxidants,antistatic agents and the like. It will be understood by those skilledin the art that such additional modifiers and additives will be selectedto avoid undesirable interaction with the resin, blowing agents andnucleating agents, and will be used at levels appropriate to theirfunction and purpose according to common practice in the foam resincompounding and formulating arts.

[0029] The improved foamable polypropylene compositions of thisinvention will further comprise a bubble nucleating agent. Bubblenucleating agents create sites for bubble initiation and desirablyinfluence cell size and minimize the occurrence of large bubbles andopen-cell structure, thereby providing particularly attractive anduniform high quality foam sheet. Use of a bubble nucleating agent incombination with a crystallization nucleating agent also furtherimproves foam processability and melt rheology, as well as desirablyenhancing important mechanical and thermal properties of the foam sheet,particularly rigidity or stiffness.

[0030] The bubble nucleating agent employed in formulating improvedcompositions useful for foam sheet extrusion according to the inventionmay be selected from the variety of inert solids disclosed in the art tobe useful as bubble nucleating agents, including mixtures of citric acidand sodium bicarbonate or other alkali metal bicarbonate, talc, silicondioxide, diatomaceous earth, kaolin, polycarboxylic acids and theirsalts, and titanium dioxide. Other inert solids disclosed in the art forthese purposes may also be found suitable. The nucleating agent willpreferably have a mean particle size in the range of from about 0.3 toabout 5.0 microns (μm), and will be present at a concentration of up toabout 5 wt. %, preferably from about 0.01 to about 5 wt. %, and morepreferably from about 0.5 to about 2 wt. % of the total weight of theformulation. At higher concentrations the cell structure becomesundesirably small; further, the nucleating agent tends to agglomerateduring processing.

[0031] A variety of compounding and blending methods are well-known andcommonly used in the art and most may be adapted to mix and compound thecomponents of foamable polypropylene formulations. Conveniently, theresin together with stabilizers and further additives and modifyingcomponents that are not thermally sensitive, whether in powder, pellet,or other suitable form, may be mixed and melt compounded using a highshear mixer, e.g., a twin-screw extruder at temperatures effective torender the resinous components molten and obtain a desirably uniformblend. Thermally sensitive components of the formulations, includingblowing agents, may be physically mixed with the resin in powder orpellet form using conventional dry-blending methods just prior tofeeding the mixture to the extruder. Plasticating the resin in acompounding extruder and feeding the additives and modifying componentsto the molten composition through a port in the extruder is alsocommonly practiced in the art. Downstream addition to the melt also maybe found particularly useful for foam sheet extrusion where a physicalblowing agent in the form of a gas is employed.

[0032] Processes for extruding foam sheet generally employ an extrusionapparatus having single or multiple extruders, which may be single ortwin screw extruders, to conduct the mixture of polypropylene resin andadditives through the plasticating and mixing steps, and provide amolten, foaming or foamable resin mass to the inlet of a sheet extrusiondie. Preferably the extrusion die will be a coathanger type sheetextrusion die wherein the inlet extends to a coathanger-shaped plenum inthe form of a relatively wide and vertically narrow cavity, elongated inthe horizontal or transverse direction (Y axis) and relatively narrow inthe vertical direction (Z axis). The resin flow direction or machinedirection may also be referred to as the extrusion axis (X axis). Theplenum is in liquid communication with an exit port or mouth extendingalong the width or Y axis of the die, forming a slit defined by dielips. Molten, foaming resin enters the die through the inlet, is spreadacross the width of the die by way of the plenum, passes between dielips and exits through the exit port or die exit in a molten orsemi-molten state as a continuous sheet. The extruded sheet will then becooled to become solidified, for example by being passed through a rollstack to cool the foam and finish the sheet. Differential roll speedsand take-up speeds may be employed to draw the foam sheet, orienting thecrystalline polypropylene and achieving a final form and thickness forthe sheet.

[0033] Turning now to the drawings, it may be seen in FIG. 1 that atypical coathanger die, generally designated by the reference numeral10, for extruding thermoplastic sheet will comprise a first or lowerhalf 1 and a second or upper half 2, indicated in phantom. Assembled inopposed relationship, the halves form cavity 3. Molten foaming resinenters the die through inlet 4 and flows into coathanger-shaped plenum5. Plenum 5 spreads the molten resin uniformly across the width of thedie as it flows through the preland passage 6 to melt well 7. Adjustablechoke means (not shown) may be included to provide control of resinflow, and any differences in pressure still remaining may be evened outby melt well 7. The molten resin continues through planar extrusionpassageway or die land 9 defined by the opposing, spaced apart faces oflower die lip 11 and upper die lip 12, exiting the die through exitopening 8 and forming a sheet.

[0034] It will be understood that the die body may include passages forheating and cooling, and further that clamping and fastening means andmeans for assembling the die to the extruder, also required, have beenomitted from the drawings for clarity.

[0035] In FIG. 2 it will be seen that Prior Art sheet extrusion die 20includes assembled upper and lower halves 21 and 22, together definingcavity 23, and upper and lower die lips 14 and 15. As described above,molten foaming resin will be supplied under pressure by extruder means(not shown) to die cavity 23 through inlet 24, in fluid communicationwith exit opening 28. Flowing into the coathanger plenum 25, and dammedby the narrowing of the cavity at preland passage 26, the melt stream isspread across the width of the die by plenum 25 and fills melt well 27.The molten resin, further regulated by adjustable choke means 61, flowsfrom melt well 27 and passes through extrusion passageway or die land29, exiting the die through exit opening 28 as continuous foam sheet.

[0036] Either or both of die lips 14 and 15 may be made fixed orremovable as desired. As shown in FIG. 2, lower die lip 14 is maderemovable, secured to the lower body portion 12 by a plurality of boltsor other suitable fastening means, while upper die lip 15 is fixed.Upper die lip 15 may be provided with adjusting means 62 as shown, forthe purpose of adjusting the gap between the die lips at exit opening28.

[0037] In prior art sheet extrusion dies such as shown in FIG. 2, theopposing faces of the upper and lower die lips 14 and 15 that makecontact with the molten resin stream are planar and substantiallyparallel, defining a smooth extrusion passageway or die land 29 having asubstantially uniform height or thickness. The continuous foam sheetexiting the die lips in a molten or semi-molten state will havesufficient internal pressure to undergo further expansion on exiting,reaching a final thickness and surface condition when the temperature ofat least the outer skin falls below the crystallization temperature andsolidifies.

[0038] To complete the finishing of the extruded foam sheet,conventional sheet extrusion processes generally employ a finishing rollstack (not shown) which may contain chilled rolls to further cool thesheet. Optionally, cooling means such as an air stream may be providedat the exit port to quickly cool the surfaces of the emerging foamsheet. Though initially shaped by the exit opening 28, the finalthickness of the foam sheet will thus depend in part on the coolingmeans employed, the roll gap within the finishing roll stack, and theratio of the extrusion rate to the take-up rate, which may be selectedto draw the sheet as desired and orient the crystalline polypropylenecomponent of the foam sheet.

[0039] In FIG. 3, showing an embodiment of an improved sheet extrusiondie according to the invention, the improvement over the prior art dieof FIG. 2 will be seen to reside in the modification of die lip 16which, together with upper die lip 17, will provide a die land 39 withwidening aperture downstream along the extrusion axis from near meltwell 37 to exit opening 38, thereby providing expansion zone 65. Moltenfoaming resin flowing from melt well 37 of die 30 enters expansion zone65, expands under the influence of the pressure drop and exits the exitopening or die port as a continuous foam sheet. Foam expansion to agiven thickness is substantially completed within the die land. Byavoiding any significant further expansion after exiting the die port,surface banding, corrugation and similar surface markings will besignificantly reduced and may be entirely eliminated. Such defects arecommonly seen in foam sheet extruded using prior art extrusion dieswithout such an expansion zone such as, for example, the die shown inFIG. 2.

[0040] It may be found desirable to aid the cooling of the foam sheetwhile in the expansion zone 65, thereby speeding crystallization, andprovision may be made for including cooling means in the upper and lowerdie lips 16 and 17 near exit opening 38 for this purpose. Finishingrolls, draw rolls and the like may be employed as desired to completethe process and finish the foam sheet to a final thickness and form.

[0041] In FIG. 4, showing an alternative embodiment according to theinvention, die land 49 is provided with a widening aperture downstreamalong the extrusion axis by modification of lower die lip 18 and upperdie lip 19, thus providing an expansion zone 65 whereby foam expansionto a given thickness may be substantially completed within the die land.

[0042] The geometry of extrusion dies will be designed for particularconditions characterized from the rheology of the melt. It will bereadily understood that placing particular numerical values on thevariation of die gap geometry in the manner set forth above would beunduly limiting in that the die opening or die gap is a function of thedesired final foam product thickness as well as many other factors. Ifone skilled in the art were to determine the proper die geometry andsize, and the process throughput rate to produce a given product, theappearance and uniformity of the sheet product are subject toimprovement as described as a function of the modifications of the dielip according to the invention.

[0043] In defining a complete, improved process of producing a foamedpolypropylene sheet according to the invention, one must take intoaccount the die gap or opening and the length of the die land orextrusion passage, as well as operating parameters including throughputrate, product size and bulk density. The geometry parameters are notdetermined independent of the operating parameters.

[0044] Turning to FIG. 5, wherein the die land of FIG. 4 is shown inenlarged, fragmentary, view, it will be seen that the widening apertureof die land 49 may be further characterized by the height or thicknesst₁ of the passage at die exit opening 48 and the height or thickness t₂at the narrow point, i.e. the minimum height within the die land.Generally, the initial thickness of the extruded foam sheet will bedetermined substantially by the height of the exit opening 48. Hence,thickness t₁ may be as small as 0.01 inch (0.025 cm) to as great as 0.2inch (0.5 cm) or greater, depending upon whether the die is intended forthe production of thin, dense, expanded film-like sheet or thicker foamsheet, and may be even greater where the die will be used in producingfoam board or the like. To be suitable for the production of foamedsheet having acceptable surface appearance according to the invention,the geometry may be characterized in part by the difference (t₁-t₂),i.e. the difference between the height or thickness t₁ of the passage atexit opening 48 and the height or thickness t₂ at the minimum heightwithin the die land, which will lie in the range of from about 0.004 toabout 0.10 inch (0.01-0.25 cm). Alternatively described, the geometry ofthe die land for producing suitable foam sheet over the range ofinterest may be characterized by the ratio t₁/t₂ between the height orthickness t₁ of the passage at exit opening 48 and the minimum heightwithin the die land t₂, which will be in the range of from about 2 toabout 5.

[0045] The geometry of the die land suitable for producing foam sheetaccording to the invention may be further characterized by length l₁ ofdie land or passageway 49, determined from the downstream edge of meltwell 47 to said exit opening, and length l₂ of the widened portionforming expansion zone 65. Generally, the length of initial, narrowportion of the die land, defined by the difference (l₁-l₂), i.e. by thedifference between the length of die land 49 and the length of theexpansion zone 65, will lie in the range of from about 0.125 inch toabout 0.75 inch (0.3-1.9 cm).

[0046] These parameters will be found to serve generally for the designof improved extrusion dies to meet the requirements for producingextruded foam sheet according to the invention over a wide range ofproduction volumes, including the embodiment shown in FIG. 3.

[0047] In the operation of the foam sheet extrusion apparatus, upper dielip adjustment means may be employed to modify the die gap or openingwithin a narrow range to further control sheet thickness. The improvedsheet extrusion die according to the embodiment of FIG. 4 may beoperated using a take-up rate selected to remove the foam sheet from thedie prior to expanding to the full height t₁ of the die exit opening 48,allowing the operator to adjust and maintain sheet thickness over arange limited by the minimum and maximum heights t₁ and t₂ within theaperture of expansion zone 65.

[0048] Foamed polypropylene sheet extruded employing the improvedextrusion die according to the invention will have improved surfaceappearance, with little or no surface banding, corrugation or similarsurface markings, and may be further characterized as having a low levelof surface roughness that is uniformly distributed, giving the foamsheet a pleasing and acceptable cosmetic appearance.

[0049] Surface roughness and uniformity in surface roughness may bedetermined by image analysis and thus quantified, thereby providing anumerical basis for distinguishing acceptable from unacceptable foamsheet. More particularly, rough surfaces reflect light nonuniformly;areas of a surface that are smooth are more reflective, and in a grayscale photomicrograph of a surface appear white or light gray. Areascontaining defects such as the voids and indentations that formcorrugation bands will scatter light and thus appear in a gray scalephotomicrograph as dark gray or black. When digitized using computerizedimage analysis methods and translated to a binary, black/white image,the average surface roughness of the foam sheet will be related to theamount of black in the binary image, determined as a fraction of thetotal surface area.

[0050] The uniformity of the distribution of roughness over the surfacemay be also determined from the photomicrograph, again by imageanalysis. The uniformity of surface roughness will be related to thevariation in roughness over the area of the photomicrograph, determinedby examining equal areas of the image, averaging the roughness for eacharea, and then obtaining the standard deviation. The surface havinggreatest uniformity will be the surface having the lowest standarddeviation.

[0051] Foam sheet with a low average surface roughness, uniformlydistributed over the surface as reflected by a low standard deviation inroughness over the examined area generally will be considered to have anacceptable appearance.

[0052] It will be understood that improved sheet extrusion diesaccording to the invention may also be found useful for the extrusion ofunfoamed polypropylene sheet, or of multilayered foam sheet or foam coresheet, and further that foam extrusion dies having a die land includingan expansion zone according to the teachings hereof may be made inalternative configurations, including improved annular dies and inconfigurations suitable for the extrusion of foam board and plank, andin profile extrusion of shaped foam structures with improved surfaceappearance.

[0053] The methods and processes of this invention, and the formulationsand improved extrusion die employed therein, may be used for theproduction of improved foamed polypropylene over a wide range ofdensities including low density, flexible foam for packaging uses, andfor the manufacture of foamed polypropylene plank and board withthicknesses greater than ¼ inch (0.6 cm) to as great as 1 inch (2.5 cm)or more. Rigid, foamed polypropylene sheet having a thickness in therange of from about 10 mils (0.25 mm) to about 250 mils (6 mm),preferably from about 20 mils (0.5 mm) to about 80 mils (2 mm), and withfoam densities in the range of from about 0.4 g/cm³ to about 0.8 g/cm³,preferably from about 0.45 g/cm³ to about 0.8 g/cm³ and still morepreferably from about 0.5 to about 0.75 g/cm³, will be particularlypreferred. The rigidity of the high density foam sheet of this inventionis reflected in the high tensile modulus of the foam sheet, generallyabove about 150,000 psi (1000 MPa), more particularly in the range offrom about 150,000 psi to about 300,000 psi (1000-2000 MPa).

[0054] At lower densities, foam polypropylene sheet is more flexible andlacks the rigidity desired for rigid food packaging uses and the like.Thus, the lower density polypropylene foam sheet disclosed in U.S. Pat.No. 5,149,579 is characterized by patentees as having tensile andflexural moduli values in the range of from 10,000 to 50,000 psi (70-340MPa). At foam densities above about 0.8 g/cm³, foam polypropylene sheetwill lack the thermal insulation characteristics desired for manyapplications.

[0055] Thin, rigid foam sheet produced according to the invention willalso have good thermal insulating properties, with a thermalconductivity in the range of from about 0.08 W/m°K to about 0.15 W/m°K.Foam sheet having a high degree of rigidity in combination with goodthermal insulating properties will be particularly attractive andimportant for acceptability in food service applications, for example,for use in the manufacture of cups and similar containers for handlinghot or chilled liquids, and where food must be maintained attemperatures significantly above or below ambient temperature for someperiod of time. Foam intended for these uses will preferably have athermal conductivity below about 0.14 W/m°K and more preferably belowabout 0.11 W/m°K.

[0056] Polypropylene foam sheet produced from the improved polypropylenecompositions according to the invention may be used in a conventionalthermoforming operation to form rigid and semi-rigid articles.Typically, articles are formed from sheet having a thickness of fromabout 10 mils (0.25 mm) up to 200 mils (5 mm) or above. A thermoformedarticle typically ranges from about 20 to 80 mils (0.5-2 mm). Generally,processes for thermoforming foam sheet include the steps of heating thefoam sheet to a temperature where it is deformable under pressure orvacuum, supplying the softened foam sheet to a forming mold, and coolingthe foam sheet to form a rigid or semi-rigid article having the shape ofthe mold. To avoid collapsing the foam structure of the sheet, thetemperature employed in the heating step will fall in a narrow rangewhich does not exceed the melt temperature of the resin. The processingwindow or temperature range for thermoforming, and particularly theupper temperature limit, may be conveniently assessed by athermomechanical analysis procedure whereby a sample of the sheet isheated while monitoring the change in thickness of the sheet as afunction of temperature, using a thermomechanical analyzer probe. Uponreaching and then exceeding the upper limit of the processing range, thethickness of the sheet will be observed to rapidly decrease as the foamstructure collapses and the probe penetrates the sheet. Generally,extruded foam sheet comprising polypropylene may be processed with goodretention of foam structure at temperatures of from about 130° to about145° C., and particular formulations may be found to be processable attemperatures as great as 150° C. while retaining foam structure.

[0057] In another aspect of the invention, a surface layer may beapplied by co-extrusion techniques. Typically, top and bottom surfacelayers with thickness ratios of the layer to the foam core of about1:1000 and preferably 1:2000 or above may be used. Preferably, thesurface layer is a propylene polymer with a similar composition (exceptfor blowing agents) to the foam core, although any compatible propylenepolymer may be used. Also, if desired a barrier resin layer also may beapplied such as polyethylene or ethylenevinylacetate polymer. Anadvantage of using co-extruded surface layers is incorporating pigmentsor other specialized additives to the surface layers. Since the amountof surface layer is much smaller than the foam core, the use of pigmentsor other additives is minimized. This may be beneficial in recycling thearticle.

[0058] Extruded polypropylene foam sheet having improved surfaceappearance according to the invention has application in a wide varietyof physical shapes and forms in addition to molded goods. Rigid andsemi-rigid foams, including molded and laminated products preparedtherefrom, not only possess good physical properties and excellentchemical resistance at room temperature, but they retain their strengthand good performance over a wide range of temperatures and for longperiods of time. Molded articles formed from the preferred foamcomposition of this invention have markedly improved surface appearanceand may be particularly useful in food packaging where appearance andcosmetic considerations are highly important to consumer acceptance.Examples include plates, cups, trays, and containers such as fortake-out food and home meal replacement items. Since these articles aremade from propylene polymer with a relatively high softening point, thearticles typically may be used in a microwave oven. The foam sheet andmolded articles may also find wide use in applications where mechanicalstrength, rigidity and thermal insulation are important considerations,such as in durable goods and appliance components, and in medical andplumbing applications where resistance to hot, humid environments may beparticularly important, as well as in safety equipment and protectivegear.

[0059] The invention will be better understood by way of considerationof the following illustrative examples and comparison examples, whichare provided by way of illustration and not in limitation thereof. Inthe examples, all parts and percentages are by weight unless otherwisespecified.

EXAMPLES

[0060] The PP resins employed in the following examples were preparedusing the Amoco Gas Phase Process. The process is disclosed generally in“Polypropylene Handbook” pp. 297-298, Hanser Publications, NY, 1996, andis more fully described in U.S. Pat. No. 3,957,448, the teachings ofwhich are incorporated herein in their entirety by reference thereto,and in “Simplified Gas-Phase Polypropylene Process Technology” presentedin Petrochemical Review March, 1993.

[0061] PP resins are initially produced in powder form. The resin powdermay be used directly, or may be first compounded and pelletized bystrand extrusion using a compounding extruder, and then chopping thestrand. Pelletizing may be accomplished according to standard practice,for example by dry-blending dried resin with such stabilizing componentsand additives as may be required and feeding the blend to a ZSK-30twin-screw extruder. The polymer, extruded through a strand die intowater, is then chopped to form pellets. PP resins are generallycharacterized by molecular weight; among the measurements employed fordescribing resin molecular weight, in addition to resin viscosity, isthe resin Melt Flow Rate or MFR. Generally, molecular weight isinversely related to MFR.

[0062] The component materials employed in following examples, and theabbreviations therefor, include:

[0063] Polypropylene Resins

[0064] PP-1: propylene homopolymer, powder, MFR=2 g/10 min.

[0065] PP-2: propylene homopolymer, powder, MFR=1 g/10 min.

[0066] PP-3: propylene homopolymer, pelletized, MFR=2 g/10 min.

[0067] PP-4: propylene-ethylene copolymer, pelletized, MFR=2 g/10 min.

[0068] Crystal Nucleating Agents

[0069] Nucl-1: sodium benzoate

[0070] Nucl-2: organic sodium phosphate, obtained as MARK NA-11 fromAdeka Argus Chemical Company

[0071] Nucl-3: dimethyldibenzylidene sorbitol, obtained as Millad 3988from Milliken Chemical Company

[0072] Blowing Agents

[0073] FPE-50: Proprietary sodium bicarbonate-based blowing agent,obtained as SAFOAM FPE-50 from Reedy International Corporation, added asconcentrate, 50 wt. % active.

[0074] CF40E: Proprietary sodium bicarbonate-based blowing agent,obtained as BI CF40E from BI Chemical Company, added as concentrate, 40wt. % active.

[0075] H40E: Proprietary sodium bicarbonate-based blowing agent,obtained as BI H40E from BI Chemical Company, added as concentrate, 40wt. % active.

[0076] Bubble Nucleating Agent

[0077] Talc: talc having a 0.8 micron mean particle size, maximumparticle size 6 microns, obtained from Specialty Minerals Inc. asMicrotuff AG-609 and AGD-609 grades.

[0078] Test methods employed in evaluating the foam sheet produced inthe following Examples include:

[0079] Melt Flow Rate (MFR) was determined by ASTM D1238, Condition L(230° C., 2.16 Kg load).

[0080] Density was determined according to ASTM D1622.

[0081] The crystallization temperature (Tc) and melting temperature (Tm)were determined by differential scanning calorimetry followingsubstantially the procedures of ASTM E-793.

[0082] Thermal conductivity was determined using a C-matics modelTCEM-DV instrument from Dynatech Corporation following test procedurespublished by the manufacturer.

[0083] Tensile testing was carried out substantially according to theprocedures of ASTM D638.

[0084] Evaluation of cell dispersion and size was made by scanningelectron microscopy (SEM) on cross sections microtomed from PP foamsheet. Specimens were taken along the transverse and machine directions.The cut specimens were mounted on SEM stubs, coated with Au/Pd, andexamined on a Hitachi S-4000 scanning electron microscope in secondaryelectron imaging mode, accelerated voltage 10 kV. Cell distribution wassubjectively rated visually on a basis of excellent, very good, good,fair, poor and very poor uniformity.

Examples 1-6

[0085] In Examples 1-6 and Control Examples C-1-C-3, powdered propylenehomopolymer resin having an MFR of 1.0 (PP-1), was compounded andextruded into foam sheet substantially by the following generalprocedure. The formulations are summarized in the following Table I,wherein all components are in wt. % based on total weight of theformation; balance of the formulation is PP-1. Examples 1-6 will be seento comprise a bubble nucleating agent (talc) and a crystallizationnucleating agent; the formulations of Control Examples C-1-C-3 containonly a bubble nucleating agent, without a crystallization nucleatingagent.

[0086] Polypropylene powder and the indicated amount of additives otherthan blowing agent were dry blended in the amounts indicated, fed to aZSK-30 compounding extruder, melt mixed and extruded to form pellets.The resin pellets were dry-blended with the indicated amount of blowingagent and gravity fed through the hopper to a 2.5 inch (6.4 cm)single-screw NRM extruder, having a 24/1 L/D screw, with barrel and dieheating. The barrel of the extruder was maintained at a temperature inthe range of about 360° to about 380° F. (182-193° C.). The molten,foaming mixture was extruded through a 12 inch (30.5 cm) widthcoathanger die maintained at 380° F., pulled in an S-wrap through athree-roll finishing roll stack maintained at a temperature ofapproximately 130° F. (54° C.), located approximately 6 inches (15 cm)from the die exit, and taken up on a winder. For the purposes of theseexamples, the take-up speed employed was selected to provide onlynominal drawing and orientation of the polypropylene foam in the machineor flow direction.

[0087] An improved extrusion die substantially as shown in FIG. 3 andhaving a ratio t₁/t₂=1.2 and a ratio I₁/I₂=3.5, was employed forextruding the foam sheet of Examples 1-6 and C-1-C-3. Foam sheet foreach of the formulations was extruded under low shear conditions andalso under high shear conditions to demonstrate the effect of mixing oncell distribution and uniformity. A low extruder screw speed of 86 rpmwas employed for low shear mixing, and a higher extruder speed of 120rpm for high shear mixing.

[0088] Samples of the foam sheet were taken for determination ofmechanical and thermal properties as described below.

[0089] All formulations set forth in the following Table I also contain0.08 wt. % Irganox 168 and 0.04 wt. % Irganox 1010 stabilizers, fromCiba Geigy. Examples 2-4 contain 0.02 wt. % Kyowa DHT4A grade ofsynthetic hydrotalcite, obtained from Mitsui, to neutralize acidiccatalyst residues. The resin component of these formulations (balance to100 wt. %) is PP-1 propylene homopolymer. All formulations were combinedwith 0.5 pbw (active) SAFOAM FPE-50 blowing agent per hundred partsformulation before extruding. TABLE I Exam- ple: 1 2 3 4 5 6 C-1 C-2 C-3Talc 0.6 0.75 0.9 1.1 0.75 0.75 0.6 0.75 0.9 (%) Nucl-1 0.1 0.1 0.1 0.1— — — — — (%) Nucl-2 — — — — 0.06 — — — — (%) Nucl-3 — — — — — 0.17 — —— (%) MFR 2.63 2.97 3.47 3.11 3.09 3.10 2.93 2.79 2.95 (g/10 min.) Tc122 124 124 125 128 121 121 121 121 (° C.) Tm 160 160 160 160 160 161161 160 160 (° C.)

[0090] The presence, as in Examples 1-6, or absence, as in ControlExamples C-1-C-3, of a crystallization nucleating agent is withouteffect on the melting temperature Tm. Crystallization temperature Tc forthe uncompounded polypropylene employed for these Examples, PP-1, 127°C.; however, as will be seen in the Control Examples C-1-C-3, Tc isreduced to about 121° C. by the addition of talc. Thus, nucleatingagents Nucl-1 and Nucl-2 serve to overcome the effect of talc oncrystallization temperature Tc, and Nucl-2 provides a furtherimprovement in Tc. Compare Tc for Examples 1-5 with C-1-C4.

[0091] Sorbitol compounds, described in the art as networking agentssuitable for nucleating crystallization in polyolefins, appear toprovide little improvement in crystal melt temperature for foamableformulations according to the invention based on polypropylene.

[0092] Density and thermal conductivity determined for samples of foamsheet, extruded at low and at high shear conditions for each of theformulations as described above, are summarized in Table II, below.TABLE II Shear Shear Low Thermal High Thermal Ex. Density cond. Densitycond. No. Kg/m² W/m° K. Cells Kg/m² W/m° K. Cells 1 0.71 0.15 v. good0.63 0.11 v. good 2 0.90 0.19 poor 0.83 0.14 poor 3 0.79 0.17 fair 0.670.12 good 4 0.83 0.15 fair 0.72 0.13 fair 5 0.75 0.15 good 0.78 0.14good 6 0.78 0.13 fair 0.74 0.13 fair C-1 0.71 0.12 fair 0.74 0.12 fairC-2 0.75 0.13 fair 0.67 0.12 fair C-3 0.75 0.12 good 0.71 0.11 good

[0093] The effect of high shear mixing in the extruder is to increasemelt temperature, thus improving cell distribution and reducing averagecell size. It will be apparent from comparing the densities of foamsproduced in low shear conditions with the corresponding foams producedunder high shear conditions that high shear mixing conditions generallyprovide a lower density foam with more uniform cell distribution asreflected in better insulating properties (reduced thermalconductivity).

[0094] The mechanical properties for low shear and high shear foam sheetof Examples 1-6 and Comparison Examples C-1-C-3 were also determined.The tensile properties are summarized in the following Table III.Samples were tested in the machine direction. TABLE III Low Shear HighShear E Tensile E Tensile Ex. Tensile Yield Yield Mod. Tensile YieldYield Mod. No. Kpsi (MPa) % Kpsi (MPa) Kpsi (MPa) % Kpsi (MPa) 1 2.83(19.5) 4.3 212 (1460) 2.24 (15.4) 5.3 161 (1110) 2 4.52 (31.2) 5.1 289(1990) 4.09 (28.2) 5.9 274 (1890) 3 3.95 (27.2) 4.5 266 (1830) 2.78(19.2) 5.5 198 (1370) 4 3.60 (24.8) 4.2 253 (1740) 2.44 (16.8) 5.2 181(1250) 5 3.42 (23.6 4.4 241 (1660) 3.58 (24.7) 5.5 236 (1630) 6 3.21(22.1) 4.2 246 (1700) 3.58 (24.7) 7.9 194 (1340) C-1 3.03 (20.9) 5.5 208(1430) 3.33 (23.0) 5.4 227 (1570) C-2 2.98 (20.5) 4.9 195 (1340) 2.98(20.5) 5.9 198 (1370) C-3 3.33 (23.0) 5.0 217 (1500) 2.98 (20.5) 6.3 189(1300)

Examples 7-12

[0095] In the following Examples 7-12, the formulations summarized inTable IV were compounded and extruded substantially as described forExamples 1-6, using a screw speed of 120 rpm to assure good mixing. Animproved extrusion die, substantially as shown in FIG. 3 and having aratio t₁/t₂=1.2 and a ratio l₁/l₂=3.5, was employed for extruding thefoam sheet.

[0096] The formulations also contain 0.08 wt. % Irganox 168 and 0.04 wt.% Irganox 1010 stabilizers, from Ciba Geigy. The balance of theformulation (to 100 wt. %) is the indicated resin component. Theformulations were combined with 0.5 pbw (active) blowing agent perhundred parts formulation before extruding. TABLE IV Example: 7 8 9 1011 12 PP PP-2 PP-1 PP-1 PP-3 PP-2* PP-2* Talc wt. % 1 — 1 1 0.75 0.75Nucl-1 wt. % 0.1 0.1 0.1 0.1 — — Nucl-2 wt. % — — — — 0.08 0.08 BlowingFPE-50 FPE-50 FPE-50 FPE-50 CF40E H-40E Agent: Thickness mil 35 40 42 5335 38 mm 0.89 1.02 1.07 1.35 0.89 0.97 Cell size, ave. 90 120 120 160130 210 μm Cell dispersion: good v. good fair good v. good poor SEMSurface — — — no no voids, appearance: voids, voids, poor v. good excl.Thermal cond. 0.11 0.12 0.12 0.13 0.11 0.11 W/m ° K. Density g/cm³ 0.650.65 0.73 0.66 0.62 0.56

[0097] It will be seen that not all blowing agents provide equivalentresults, the H-40E blowing agent giving foam with poor cell dispersionand large, irregular, open and interconnected cells when viewed by SEM(Example 12). Although the surfaces of the foam sheet of Examples 7-12had no significant banding, the foam sheet of Example 12 had a roughsurface, with voids; apparently, these defects were the result of usinga less effective, coarser particle foaming agent. The cells of Examples7-11, viewed by SEM in cross section taken in the transverse direction,appeared to be essentially circular in cross section. Viewed in crosssection along the machine direction, the cells of these foams will beseen to be elongated along the direction of flow.

[0098] Foam sheet having thermal conductivity below about 14 W/m°K,preferably below about 11 W/m°K, are particularly desirable for use infood service applications, particularly in the production of cups andsimilar articles for use in storing and serving hot or cold foods. Forcomparison, the thermal conductivity of extruded unfoamed, stabilizedPP-2 polypropylene sheet having a nominal thickness of 40 mils (1 mm) is0.20 W/m°K. The thermal conductivity of stock used for producing papercups is about 0.12 W/m°K, while for Styrofoam cup stock the value isabout 0.09 W/m°K.

Comparison Example C-4

[0099] For comparison purposes, foam sheet was extruded using a priorart die and the foamable polypropylene formulation of Example 11, setforth in Table IV. The prior art extrusion die was substantially asshown in FIG. 2. The foam sheet had an average thickness of 35 mils (0.9mm) and an average density of 0.62 g/cm³.

[0100] The foam sheet of the Comparison Example C-4 was seen to havepoor cell dispersion when viewed by SEM and, on visual inspection, thesurface of the foam sheet was visibly flawed, with bands running in themachine direction, defects typically seen in foam sheet extruded fromcommercial, unmodified polypropylene resins using prior art dies.

[0101] As noted in Table IV above, when inspected by SEM, the surfaceappearance of the invented foam sheet of Example 11 was seen to beexcellent, with no observable surface voids. On visual inspection, thefoam sheet of Example 11 had no visible banding or other significantsurface defects.

[0102] Surface appearance of various foam sheet specimens may be furthercompared by rating the surface roughness and uniformity for each usingroughness parameters obtained through image analysis of photomicrographsof the surface of the sheet. Specimens of sheet suitable for rating areextruded substantially as in Examples 10 and 11; for comparisonpurposes, representative specimens of unacceptable foam sheet areprovided as described in the Comparison Example C-4 and rated.Representative specimens of foam sheet, summarized in Table V asExamples A-C, are produced to provide further comparisons by extrudinggeneric polypropylene resins using prior art processes as in theComparative Example C-4.

[0103] In making the ratings, an area of the sheet surface approximately5 mm×16 mm was viewed by SEM and photographed. The examined area wasselected to include obvious surface roughness, together with acorrugation, line if one existed. Visual appearance ratings were thendetermined by image analysis of 16 equal areas, each 1 mm wide by 5 mmin length, selected to run parallel to the corrugation line. Thefraction (%) of dark areas for binary images of each of the strips, thestandard deviation (%) in dark area for the 16 strips, and the averagedark area (%) for the total area examined, were determined. The resultsof these evaluations are summarized in Table V.

[0104] Average Roughness is related to the % of dark area for the totalarea examined; high values indicate the presence of more voids andindentations, i.e. a rougher appearing surface.

[0105] Variation in average roughness between areas of the surfaceindicates nonuniform distribution of cells, voids and indentations onthe surface. For a completely uniform surface, whether rough or smooth,there would be little variation in average roughness between variousareas of the surface.

[0106] Standard deviation is related to the uniformity of the surfaceroughness, and indirectly to the uniformity of internal celldistribution; low values indicate more uniformly distributed cells,voids and indentations on the surface, and a more uniform distributionof cells internally.

[0107] The criteria for rating and indexing surface roughness anduniformity are summarized in tabular form as follows: RoughnessUniformity Rating (Ave., %) (Std. Dev., %) 1 <10 3 2 10-20 3-4 3 20-304-5 4 30-40 5-6 5 40-50 6-7 6 50-70  7-10 7 >70 >10

[0108] TABLE V Surface Roughness and Uniformity Ratings ofRepresentative Specimens of Foamed Polypropylene Sheet Specimenaccording to Ex.: 10 11 C-4 A B C Roughness, Ave. % 24.6 74.6 59.2 20.638.4 14.3 Roughness rating: 3 7 6 3 4 2 Std. Deviation % 3.2 3.7 22.04.4 5.2 7.0 Uniformity rating: 2 2 7 3 4 6 min. % 18.4 68.0 29.3 14.126.2 5.8 max. % 32.6 79.3 94.3 30.2 47.5 30.7 Visual Appearance: B C E CD F

[0109] It will be seen that the primary characteristic of foam sheethaving an acceptable surface appearance is uniformity in thedistribution of surface roughness. When the surface roughness isuniformly distributed, whether low as in Example 10 or high as inExample 11, foamed polypropylene sheet will have good visual appearance.Very faint corrugation lines will result from a less uniformdistribution of surface defects as in Example A. The corrugations becomemore noticable and have a deleterious effect on surface appearance asdistribution becomes more nonuniform. See Example B and the ComparisonExample C-4. Conversely, low surface roughness, when distributed in ahighly non-uniform manner, provides sheet having a very undesirablesurface appearance as in Example C.

[0110] Acceptable foam sheet will thus be seen to be sheet having auniform distribution of surface roughness, reflecting an overalluniformity in internal cell distribution. For foam sheet according tothe invention, the standard deviation in surface roughness, a measure ofuniformity, will generally be less than about 6.0%, more preferably lessthan about 5.0% and most preferably below about 4.0%.

[0111] The invention will thus be seen to be directed to an improvedpolypropylene composition for use in the production of extruded foamsheet, more particularly, rigid or semi-rigid foamed polypropylene sheethaving a thickness of from about 20 to about 80 mils (0.5-2 mm) and adensity of from about 0.4 to about 0.8 g/cm³. Rigid, foamedpolypropylene sheet according to the invention will be significantlyimproved in surface appearance, with the substantial absence of thebanding and corrugation markings commonly seen in extruded rigidpolypropylene foam of the prior art. The invention may also becharacterized as directed to rigid foamed polypropylene sheet comprisinggeneric polypropylene as defined herein and having a density of fromabout 0.4 to about 0.8 g/ cm³, and a uniform distribution of surfaceroughness which may be further defined as a standard deviation in theaverage surface roughness generally below about 6%, more preferablybelow about 5.0% and most preferably below about 4.0%.

[0112] Although the invention has been described and illustrated by wayof specific embodiments set forth herein, those skilled in the art willrecognize that a variety of polypropylene homopolymer and copolymerresins including impact-modified polypropylene resins such as, forexample, those disclosed in the art based on an isotactic polypropylenecontaining a dispersed phase comprising a copolymer may also be founduseful. Still further modifications and variations in the processesemployed herein for the production of foam sheet will be readilyapparent to those skilled in the resin formulating and fabricating artand in the extrusion arts, and such variations and modifications will beunderstood to lie within the scope of the invention as defined by theappended claims.

We claim:
 1. A foamable polymer composition comprising a polypropyleneresin having a melt flow rate of from about 0.5 to about 30 g/10 min.determined according to ASTM D1238 condition L, said resin havingdispersed therein from about 0.05 to about 0.5 wt. % of acrystallization nucleating agent, from about 0.01 to about 5 wt. % of afinely divided inert solid and from about 0.1 to about 25 wt. % of afoaming agent.
 2. The foamable polymer composition of claim 1 whereinsaid crystallization nucleating agent is at least one nucleating agentselected from the group consisting of organic sodium phosphates, sodiumbenzoate and mixtures comprising (a) a monocarboxylic aromatic acid or apolycarboxylic aliphatic acid and (b) a silicate or an alumino-silicateof an alkali or alkaline earth metal.
 3. The foamable polymercomposition of claim 1 wherein said crystallization nucleating agent isselected from the group consisting of sodiumbis(4-tert-butyl-phenol)phosphate and sodium benzoate.
 4. The foamablepolymer composition of claim 1 wherein said finely divided inert solidhas a mean particle size in the range of from about 0.3 to about 5microns.
 5. The foamable polymer composition of claim 1 wherein saidfinely divided inert solid is selected from the group consisting oftalc, silicon dioxide, diatomaceous earth, kaolin and titanium dioxide.6. The foamable polymer composition of claim 1 wherein said finelydivided inert solid is talc.
 7. A foamable polymer compositioncomprising a polypropylene resin having a melt flow rate of from about0.5 to about 30 g/10 min. determined according to ASTM D1238 conditionL, said resin having dispersed therein from about 0.05 to about 0.5 wt.% of a crystallization nucleating agent selected from the groupconsisting of sodium bis(4-tert-butyl-phenol)phosphate and sodiumbenzoate, from about 0.01 to about 5 wt. % of a finely divided talchaving a mean particle size in the range of from about 0.3 to about 5microns, and from about 0.1 to about 25 wt. % of a foaming agent.
 8. Thefoamable polymer composition of claim 7 wherein said crystallizationnucleating agent is sodium benzoate.
 9. A rigid, foamed polypropylenesheet having a density of from about 0.4 to about 0.8 g/cm³, said foamedpolypropylene sheet obtained by foam extrusion of a compositioncomprising a polypropylene resin having a melt flow rate of from about0.5 to about 30 g/10 min. determined according to ASTM D1238 conditionL, said resin having dispersed therein from about 0.05 to about 0.5 wt.% of a crystallization nucleating agent, from about 0.01 to about 5 wt.% of a finely divided inert solid and from about 0.1 to about 25 wt. %of a foaming agent.
 10. The foamed polypropylene sheet of claim 9 havinga thickness of from about 10 mil to about 250 mil.
 11. The foamedpolypropylene sheet of claim 9 having a thickness of from about 10 milto about 80 mil.
 12. The foamed polypropylene sheet of claim 9 whereinsaid density is in the range of from about 0.45 to about 0.8 g/cm³. 13.The foamed polypropylene sheet of claim 9 wherein said finely dividedinert solid has a mean particle size in the range of from about 0.3 toabout 5 microns and is selected from the group consisting of talc,silicon dioxide, diatomaceous earth, kaolin and titanium dioxide. 14.The foamed polypropylene sheet of claim 9 wherein said crystallizationnucleating agent is selected from the group consisting of organic sodiumphosphates and sodium benzoate.
 15. The foamed polypropylene sheet ofclaim 9 wherein said crystallization nucleating agent is sodiumbenzoate.
 16. The foamed polypropylene sheet of claim 13 wherein saidinert solid is talc.
 17. A molded article comprising foamedpolypropylene resin, said article molded by thermoforming an extruded,rigid, foamed polypropylene sheet having a density in the range of fromabout 0.4 to about 0.8 g/cm³ and a thickness in the range of from about10 mils to about 200 mils, said foamed sheet extruded from a compositioncomprising a polypropylene resin having a melt flow rate of from about0.5 to about 30 g/10 min. determined according to ASTM D1238 conditionL, from about 0.05 to about 0.5 wt. % of a crystallization nucleatingagent, from about 0.01 to about 5 wt. % of a finely divided inert solidand from about 0.1 to about 25 wt. % of a foaming agent.
 18. An articleof claim 17 wherein the thickness is about 20 to about 80 mils.
 19. Anarticle of claim 17 on which is co-extruded surface layers of acompatible polymer.
 20. An article of claim 17 on which is co-extrudedsurface layers of a propylene polymer which contains pigment.
 21. Anarticle of claim 17 in the form of a plate, cup, tray, or container. 22.A rigid, extruded sheet having a density of from about 0.4 to about 0.8g/cm³ comprising foamed, substantially linear propylene polymer havingan essentially monomodal molecular weight distribution and a melt flowrate of from about 0.5 to about 30 g/10 min. determined according toASTM D1238 condition L, the surfaces of said sheet having a standarddeviation in surface roughness of less than about 6%.
 23. The rigid,extruded sheet of claim 22 wherein said standard deviation in surfaceroughness is less than about 5%.
 24. A molded article comprising foamed,substantially linear propylene polymer having an essentially monomodalmolecular weight distribution and a melt flow rate of from about 0.5 toabout 30 g/10 min. determined according to ASTM D1238 condition L, thesurfaces of said molded article having a standard deviation in surfaceroughness of less than about 6%.
 25. An article of claim 24 in the formof a plate, cup, tray, or container.